\section{Technology used by ontology-building tools}
Since the tools presented in this paper utilize various Semantic Web technologies such as RDF, RDF Schema and OWL, we will present those technologies in this introductory section. We expect the reader to be familiar with basic Web technologies like HTML, XML, wikis, tagging, etc. 

\subsection{RDF}
The \textit{Resource Description Framework} (RDF) is described in~\cite{wiki:rdf} as follows: ``The RDF data model is [\ldots] based upon the idea of making statements about resources (in particular Web resources) in the form of \textit{subject-predicate-object} expressions. These expressions are known as triples in RDF terminology.''

For example\footnote{Example taken from the RDF Primer W3C Recommendation (10 February 2004), available online at \url{http://www.w3.org/TR/rdf-primer/}}, the sentence ``\url{example.org} has a creator whose value is John Smith'' could be represented by an RDF statement having:
\begin{itemize}
  \item a subject \url{example.org}
  \item a predicate \url{http://purl.org/dc/elements/1.1/creator}
  \item and an object \url{http://www.example.org/staffid/85740}  
\end{itemize}
Note how URIs are used to identify not only the subject of the original statement, but also the predicate and object, instead of using the words ``creator'' and ``John Smith'', respectively.~\cite{rdfprimer}

\subsection{RDF Schema}
RDF Schema (RDFS) provides basic elements for the description of ontologies, such as \textit{rdfs:Class}, \textit{rdfs:Literal} or \textit{rdfs:Datatype} which declare that a resource is a class for other resources, literal values and the class of datatypes respectively. Other elements include \textit{rdf:type} which is a property used to state that a resource is an instance of a class~\cite{wiki:rdfs}, or \textit{rdfs:subClassOf} which allows the declaration of hierarchies of classes.

\subsection{OWL}
The \textit{Web Ontology Language} (OWL) is a family of knowledge representation languages for authoring ontologies~\cite{wiki:owl}. The languages are characterized by formal semantics and RDF/XML-based serializations for the Semantic Web and are endorsed by the \textit{World Wide Web Consortium}~\cite{w3c:owl}.

The OWL family contains many species, serializations, syntaxes and specifications with similar names~\cite{w3c:owl}. OWL sublanguages include \textit{OWL Lite}, \textit{OWL DL} and \textit{OWL Full}. \textit{OWL2} profiles include \textit{OWL 2 EL}, \textit{OWL 2 QL}, \textit{OWL 2 RL}.